The invention relates to an anti-locking brake system, which is set up in particular for traction control and vehicle dynamics control in such a way that a pump for actuating a wheel brake removes brake fluid in a controlled manner from a brake pressure generating unit and supplies it to the wheel brake.
From DE 40 35 527 A1 it is known to provide a 2-setting/2-position valve between the input side of the pump and the brake pressure generating unit. Said valve arrangement has only one actuation and/or connection setting. In said case, the flow cross section of said one actuation and/or connection setting has to be very large so that, upon entry into the directional stability control system, a rapid pressure build-up in the wheel brake is achieved via the pump, especially in order at low temperatures to achieve a good intake performance (high pumping capacity) of the pump. A large flow cross section however entails the drawbacks of loud hydraulic flow noises and severe pedal reactions, which is perceived as intrusive and far from comfortable.
Such a valve arrangement is known as a so-called xe2x80x9cnormal-closedxe2x80x9d (NC) valve. In the non-actuated state, i.e. the basic or normal position, the flow connection between the fluid connections of the valve arrangement is blocked, whereas in the actuated state the flow connection between the fluid connections is established. So that in the non-actuated state the flow connection between the fluid connections is blocked, a valve element under the action of a spring arrangement is biased so as to come sealingly into abutment with a sealing seat. In order in the actuated state to establish the flow connection between the fluid connections, it is necessary for the actuating device to summon up a force which is capable of surmounting at least the bias force of the spring arrangement so that the valve element lifts off the sealing seat.
However, in most applications it is not enough for the actuating device to summon up a force which is only slightly greater than the bias force of the spring arrangement because during operation different fluid pressures arise at the fluid connections of the valve arrangement, so that the valve element is additionally acted upon by a pressure differential force which, depending on the effective direction, is in the opposite direction to the force summoned up by the actuating device. The actuating device accordingly has to be designed in such a way that the actuating force is greater than the sum of the bias force of the spring arrangement and the maximum anticipated pressure differential force in order to guarantee reliable functioning of the valve arrangement. One drawback of this is that the actuating device has to be overdimensioned for many situations, particularly when, as in the majority of applications, in the actuated state of the valve arrangement a large flow cross section is required to prevent a throttling effect of the valve arrangement. A large flow cross section however additionally entails a very high pressure differential force, as a consequence of which a high actuating force has to be summoned up. The result is high costs and a high outlay when designing the valve arrangement. The size of the valve arrangement is also relatively large as a result.
An object of the invention is to provide an anti-locking brake system while avoiding the previously described drawbacks.
To achieve the object, according to the invention the initially mentioned anti-locking brake system is further developed in that disposed between the brake pressure generating unit and an input connection of the pump is a valve arrangement in the form of a 2-connection/3-position valve, which in a non-actuated (normal) position blocks the connection between the brake pressure generating unit and the input connection of the pump, in a first actuating position establishes a secondary flow connection between the brake pressure generating unit and the input connection of the pump, and in a second actuating position establishes a primary flow connection between the brake pressure generating unit and the input connection of the pump, wherein the primary flow connection has a larger flow cross section than the secondary flow connection.
All of the drawbacks of the previously described known arrangement (DE 40 35 527 A1) may be avoided by the solution according to the invention, which additionally provides a further actuating position with a reduced flow cross section. For, by said means, the volumetric flow from the brake pressure generating unit to the pump may be precisely metered. This applies particularly when the volumetric flow is modulated by a purposeful temporary switch between the normal position (1.1) and the first actuating position (1.2). Thus, during regulation of the brake pressure in the wheel brake the pressure level at the output side of the pump may be adapted almost to the pressure level in the wheel brake, with the added result that hydraulic noises are reduced.
Particularly suitable for use as the two/three-way valve (1) inserted between the brake pressure generating unit 31 and input side 42c of the pump 42 is the valve arrangement, which is shown in the non-actuated state in FIG. 2 and in the actuated state in FIG. 3. For the present application the valve arrangement is then energized in such a way
that given a first current for producing the first actuating position (1.1) only the further valve element 11 is actuated (secondary flow connection B),
and that given a second current for producing the second actuating position (1.2) the valve element 3 is moreover actuated (primary flow connection A).
A crucial advantage of the anti-locking brake system according to the invention is that, before the primary flow connection is established, a secondary flow connection is first established. Because of the secondary flow connection, in the situation where different fluid pressures at the fluid connections additionally exert a pressure differential force upon the valve element a pressure-equalizing process between the fluid connections is first initiated, thereby eliminating the pressure differential force so that during establishment of the primary flow connection the pressure differential force is no longer effective and may no longer have disadvantageous consequences. Thus, only a relatively low actuating force is required, with the result that the actuating device may be of an inexpensive, simple design which takes up little installation space. As a result, the flow cross section determined mainly by the primary flow connection may be relatively generous in order to rule out an undesirable throttling effect in the flow behaviour of the valve arrangement. As the secondary flow connection has only a small flow cross section compared to the primary flow connection, the pressure differential force to be surmounted during the establishment of the second flow connection is negligibly low.
The actuating device and/or the further actuating device may preferably be electromagnetically and/or hydraulically controlled. Thus, in particular, the actuating device, which actuates the valve element to establish the primary flow connection, may be electromagnetically controlled and the further actuating device, which actuates the further valve element to establish the secondary flow connection, may be hydraulically controlled. In said case, the further hydraulically actuated actuating device may be controlled by means of the pressure difference existing between the fluid connections in order to establish the secondary flow connection for the purpose of initiating the pressure-equalizing process between the fluid connections. When after elimination of the pressure differential force the primary flow connection is established by the electromagnetically actuated actuating device, the actuating force to be summoned up is likewise only relatively low. This has a particularly advantageous effect upon the design of the electromagnetic arrangement of the actuating device, because the fact that the electromagnetic force to be summoned up is relatively low on the one hand means an energy-saving and hence inexpensive current input and on the other hand enables a compact design of the components of the magnetic circuit, above all of the coil and the armature.
In a preferred form of construction of the invention, the valve arrangement comprises a first valve element, which is biased into the (normal) position by a spring arrangement, wherein a first actuating device is provided for bringing the valve element into the first actuating position, and a further valve element is provided, which is biased into the (normal) position by a further spring arrangement, and is bringable by a further actuating device into the second actuating position, so that upon actuation of the valve arrangement initially only into the secondary flow connection and then the primary flow connection exists.
According to a preferred form of construction, both the valve element and the further valve element are actuated by a common actuating device. In particular, because of the previously described advantages said common actuating device may be electromagnetically controlled. Said actuating device may comprise a two-stage plunger, which in dependence upon the armature moving the actuating device in a first stage actuates only the further valve element in order initially to establish the second flow connection, and then in a second stage to actuate additionally or exclusively the valve element, which establishes the primary flow connection characterizing the flow behaviour of the valve arrangement. Furthermore, one actuating device is saved, thereby allowing components and costs to be saved, and also a compact style of construction of the valve arrangement is achieved by virtue of the common actuating device.
In a particularly preferred form of construction, the valve element is dome-shaped and the further valve element is spherical, wherein the further valve element is disposed inside the valve element, thereby allowing the valve arrangement to be designed in a particularly space-saving manner.
The bias force of the spring arrangement is advantageously greater than the bias force of the further spring arrangement. As a result, the actuating force for establishing the secondary flow connection, which depends on the bias force of the further spring arrangement, is particularly low, above all when there are different fluid pressures at the fluid connections. The further spring arrangement may moreover be supported via the spring arrangement in order to achieve a simple and space-saving design of the valve arrangement.
From the aspects of simplicity and saving installation space, it is advantageous when the primary flow connection is determined by a sealing seat formed on the housing of the valve arrangement and by the valve element. The same applies when the secondary flow connection is formed by a sealing seat formed on the valve element and by the further valve element.
As the balance of forces is determined, on the one hand, by the actuating force to be summoned up by the actuating device and, on the other hand, by the bias force to be summoned up by the spring arrangement and, occasionally, by the pressure differential force between the fluid connections of the valve arrangement, with a view to simplicity and saving installation space the spring arrangement may be supported against the housing of the valve arrangement. The same advantage exists when the further spring arrangement is supported via the valve element.
An advantageous alternative is when the further valve element is integrally connected to the actuating device, with the result that components of the valve arrangement may be saved or may be easier to manufacture. In said case, to make the valve arrangement even more compact, it may be provided that the further spring arrangement biases the actuating device so that the further valve element occupies its first position, wherein the further spring arrangement is supported against the housing of the valve arrangement.
In a particularly preferred manner the valve arrangement according to the invention is to be used in an anti-locking brake system, which is set up for traction control and vehicle dynamics control, and to said end comprises a pump, which removes brake fluid from a brake pressure generating unit in order to supply the brake fluid to a wheel brake, wherein the first fluid connection of the valve arrangement is connected to the brake pressure generating unit and the second fluid connection of the valve arrangement is connected to the input side of the pump. Use of the valve arrangement according to the invention offers the great advantage that, on the one hand, a relatively large flow cross section is provided from the brake pressure generating unit to the input side of the pump so that the pump, above all at low temperatures, i.e. when the brake fluid behaves in a viscous manner, may deliver a relatively high volumetric flow. On the other hand, only a relatively low actuating force has to be summoned up when the valve arrangement is pressurized at the brake pressure generating unit side, which is the case upon actuation of the brake pressure generating unit by the driver via the brake pedal or upon automatic actuation of the brake pressure generating unit for preloading the pump.